The present invention generally relates to coating systems. More particularly, this invention relates to coating systems for erosion protection of components under aggressive erosion conditions, for example, components of turbomachinery including steam turbines used for power generation.
It is generally recognized that the performance of a land-based steam turbine engine is greatly influenced by the design and performance of last stage buckets operating at reduced steam pressures. Ideally, the last stage bucket should efficiently use the expansion of steam down to the turbine exhaust pressure, while minimizing the kinetic energy of the steam flow leaving the last stage. Service requirements of steam turbine buckets can be complex and demanding. Last stage buckets, in particular, are routinely exposed to a variety of severe operating conditions, including the erosive environments caused by high moisture and the carry-over from a boiler in conjunction with the high tip speeds contributing to high impact velocity of the water droplets. Such conditions can lead to serious erosion and corrosion problems with the bucket material, particularly in longer, last stage turbine buckets having vane lengths of 40 inches (about 100 cm) or greater. Thus, for some time, last stage buckets for turbines have been the subject of repeated investigations and development work in an effort to improve their efficiency under harsh operating conditions since even small increases in bucket efficiency and life span can result in significant economic benefits over the life of a steam turbine engine.
Long last stage steam turbine buckets experience higher tensile loadings and thus are subject to stresses which, when combined with an erosive environment, can be very damaging over long periods of use. Steam in the last stages normally is wet, that is, containing a higher amount of saturated steam relative to preceding stages. As a result, water droplet impact erosion of the bucket material often occurs in the last stage. Such erosion reduces the useable service life of the bucket and the efficiency of the steam turbine as a whole.
Previous approaches to solving the above-mentioned problems with longer vane lengths in last stage buckets vary widely, depending on the end use requirements. In some cases, where service demands are less severe, a conventional bucket material may be acceptable. However, in order to increase erosion resistance, a leading edge of the buckets is normally hardened through localized heat treatment (e.g., flame or induction hardening) to provide additional erosion resistance. Alternatively, an erosion resistant shielding material, such as Stellite 6, can be attached to the bucket by brazing, gas tungsten arc welding, or electron beam welding. These prior art physical attachment methods have limitations that include high cost, longer production cycles, and limited erosion resistance under aggressive operating conditions.
Another known method of manufacturing or repairing steam turbine buckets involves machining specially designed grooves at or near the leading edge for moisture removal to reduce the rate of water droplet erosion. However, this method may not offer adequate protection in high moisture or aggressive erosion environment.
The above-mentioned prior art methods may not provide lasting erosion protection under more aggressive erosion conditions expected in next generation turbines, for example, concentrated solar power turbines, that involve higher moisture contents and/or higher water droplet impact velocities than previous turbines. Additionally, these methods entail risks and limitations that would preferably be avoided or minimized, for example, weld cracking, lower hardness of erosion shields, stress corrosion cracking in the heat affected zone, high costs, and long production cycle times.
In view of the above, it can be appreciated that there are certain problems, shortcomings or disadvantages associated with the prior art, and that it would be desirable if systems and processes were available that are capable of promoting long-term resistance of components to erosion, and particularly land-based steam turbine engine components that operate in high moisture environments.